It is because MessageBox.Show() is implemented with pinvoke, it calls the native Windows MessageBox() function. Which doesn't mind getting a NULL for the lpText argument. The C# language has much stricter rules for pure .NET instance methods (like ToString), it always emits code to verify that the object isn't null. There's some background info on that in this blog post.

Because, the second call is expecting an object of "s" to satisfy a ToString() method request. so, before .Show() is called, the s.ToString() would failed with an attempt to call a method.

Interestingly, While .Show() is implemented correctly, many such methods expect non null instances to be passed in. Usually, that is when you use a NullObject pattern so that the caller should not have to deal with this kind of behavior.